The problem though is that I would rather not have uber creatures be "bosses" necessarily but rather characters in the game. Mindless giant creatures could be bosses, or enemy npc generals, or certain daedra (because they can come back into being after being killed). Otherwise, the creatures would be too powerful to be effective bosses unless you had some extremely powerful magic with which to fight the magical creature.
What I mean by uber creatures are things like trolls, ogres, Sloads at low-moderate character development, progressing to liches, spriggans, dragons(?), and Giants (nation).
Liches could be extremely powerful to the point of being near impossible to kill: when you kill an undead "lich," what if you're just killing the equivalent of a lich's sock puppet? Basically, the lich would be more like that woodland fairy from Twilight princess with all the marionettes. You could kill a weak "lich puppet," but it would take some serious work to kill the lich.
Spriggans could play a similar role with plants and roots and trees and stuff attacking you (even cooler: with Conjuration you could use the spriggan's unique abilities against enemies).
Dragons would just be impossible to kill without some sort of trickery and strong advantage.
Draugr would just be impossible to kill completely.
Each of these creatures could interact with the player for quests and such. In fact, this could be how much of magic is done until the player can join the mages guild (where do Tamriel's inhabitants get there magic from?). This would be an easy way of distinguishing different kinds of spellcasters/warriors: Berserkers might get special abilities from contact with Draugr, a mage could be a necromancer by getting power through a deal with a lich, and a druid could learn special spells by dealing with "nature spirits" such as spriggans.